The present invention concerns the production of compound thin films by deposition from the gaseous phase, this invention relating to a method for producing such compound thin films and an apparatus for carrying out the method.
Among the methods for producing compound thin films from the gaseous phase, the most important method is vacuum evaporation. This effected either directly using the particular compound as the source of the vapors or by simultaneous evaporation of the different chemical element components from different sources and thus subjecting the substrate on which the compound thin film is to be formed simultaneously to the vapors of the elements. In other words the substrate is simultaneously subject to the vapors from the evaporation of the compound or from the simultaneous evaporation of the element forming the compound.
In the first case, the major drawback resides in the decomposition of the compound into its components which makes it extremely difficult, if not impossible, to control the stoichiometry of the film that is produced, the stoichiometry generally tending to change during the course of the evaporation process.
When the gases for the formation of the thin film are supplied by simultaneous evaporation of the component elements from which the compound is formed, good stoichiometry requires extremely close control of the evaporation rates of the different compounds, or selective back evaporation of the more readily evaporating component. As in the case of evaporation from the compound itself, the nucleation properties and crystal structure of the film are inadequately controllable in the case of subjecting the substrate to the vapors from simultaneous evaporation of the components thereof.
When a single crystal substrate is used in the manner known in the art, the selective back evaporation can be made sufficiently efficient so that the growing film continues the crystal structure of the substrate. This type of procedure is known as Molecular Beam Epitaxy and is described in J. Vac. Sci. Technol., Vol. 10, No. 5, Sept./Oct. 1973, L. L. Chang et al. "Structures Grown by Molecular Beam Epitaxy".
When the completed compound is used as a source, the decomposition of the compound can be reduced in manner known in the art by means of sputtering techniques wherein the material to be transposed is detached from the source by ion bombardment. The best stoichiometry is usually attained in sputtering techniques by means of so-called bias sputtering, which is comparable to the use of back evaporation.